With the advance of digital image processing techniques, in the field of image forming apparatuses such as copying machines and printers adopting an electrophotography process or an ink-jet method, full-colored digital copying machines and digital multifunction printers which reproduce color images with high image quality are commercially available.
Examples of a document image copied by such image forming apparatuses include texts, lineal drawings, pictures, and combinations thereof. Therefore, in order to obtain a good reproduction image based on the document image, it is necessary to perform an image process in conformity to each document image.
In view of the circumstances, operational modes for selecting the type of a document, are available in an image forming apparatus, such as a text mode, a text/picture mode, and a picture mode.
However, it is very troublesome for a user to switch modes for each document. Further, if an inappropriate mode is selected, then extreme deterioration in images is often caused and therefore copies are wasteful.
As such, in order to solve the problem, there is proposed a technique for automatically judging the type of a document. Note that, a halftone printing as described below is a method used in prepress printing for example.
For example, Document 1 (Japanese Unexamined Patent Publication No. 126631/1998 (Tokukaihei 10-126631; published on May 15, 1998) discloses a technique for judging an ink-jet printing document outputted by an ink-jet printer, as well as judging the conventional type of a document such as a copied document, a silver halide print, or a halftone printing document. Namely, a characteristic regarding a color reproduction area and a characteristic regarding a texture are extracted from digital color signals obtained by reading out from the document, and then it is judged whether the document is an ink-jet printing document or not based on the characteristics.
To be specific, the ink-jet printing document has a characteristic of higher saturations of cyan and magenta than a general halftone printing document. Such a characteristic is extracted as the difference in the color reproduction area.
Further, an ink-jet printer uses an error diffusion method or a dither method to process image data. This causes the ink-jet printing original document to have a characteristic in which a texture made of dots exists on a white background of a paper. Therefore, although there is a small difference in the color reproduction areas between the ink-jet printing original document and an original document written with a fluorescent pen (namely, although the ink-jet printing document has little difference from the color reproduction areas of fluorescent blue and fluorescent purple components in terms of a signal value read by an input apparatus such as a color scanner, extraction of a characteristic regarding the texture allows the ink-jet printing original document to be discriminated from the document written with the fluorescent pen.
In Document 1, the ink-jet printing document is recognized and an appropriate process is performed with respect to the document thus recognized, thereby improving image quality. However, there are various ink-jet printing original documents printed with various resolutions, and there are various color materials, output papers, and halftones (error diffusion or dither). As such, it is difficult to recognize all ink-jet printing original documents by using conventional methods. Even if various ink-jet printing documents are recognized, it is difficult to reproduce all the ink-jet printing original documents in a good condition by the use of a single original document mode (single process).
For example, assume that an original document is prepared by outputting an image onto a paper exclusive to picture by the use of an ink-jet printer having an extremely high resolution (approximately 2800 dpi (dot per inch)). If the ink-jet printing original document thus prepared is read by an input apparatus having a normal resolution (approximately 600 dpi), then it is impossible for the input apparatus to read each dot. This will cause the result of reading which is the same as that of a photographic-picture. Under the circumstances, a better reproduction image can be obtained by processing the ink-jet printing document in the same way as a photographic-picture original document, rather than by processing the ink-jet printing original document in a way exclusive to the ink-jet printing original document.
Further, in a case of an ink-jet printing original document prepared by outputting an image onto an ordinary paper or a recycled paper by the use of an ink-jet printer having a low resolution (600 dpi or less), it is possible to read almost all dots. Further, such an ink-jet printing original document thus prepared has an equivalent range of color reproduction (color gamut) to a halftone printing original document. As a result, it is difficult to discriminate the ink-jet printing original document from the halftone printing original document. Namely, the ink-jet printing original document has substantially the same characteristic to be recognized as that of the halftone printing document. Under the circumstances, a better reproduction image can be obtained by processing the ink-jet printing document in the same way as a text printing picture document or a printing picture document (halftone printing picture).
Further, in a case of an ink-jet printing original document such as texts or graphs, a better reproduction image can be obtained by processing the ink-jet printing original document in the same way as a text original document.
On the other hand, an ink-jet printing original document becomes similar to a photographic-picture original document as the number of dots is increased, and the ink-jet printing original document becomes similar to a halftone printing original document as the number of dots drops. Note however that there is an ink-jet printing original document having an intermediate nature between the picture original document on a photographic printing paper and the halftone printing original document. Examples of the ink-jet printing document having the intermediate nature include (i) an original document which is prepared by an ink-jet image forming apparatus having a resolution between a low resolution and a middle resolution, which apparatus forms an image with high density ink so that high density areas and low density areas are mixed with each other, and (ii) an original document which is prepared by the image forming apparatus which forms an image with a low density ink so that low density areas are entirely formed.
When either a process suitable for a halftone printing original document or a process suitable for a photographic-picture document is performed with respect to the ink-jet printing original document having the intermediate nature, a deterioration in image quality such as tone distortion is caused. Note that, this kind of problem occurs not only in the ink-jet printing original document but also in other documents such as an original document outputted from based on the electrophotography method.
In view of the circumstances, a technique for exactly estimating the type of an original document image (input image) is demanded. This is because it is necessary to carry out a process suitable for the type of image so that an image with high quality can be obtained, when processing an original document image read by a scanner.